jwalgemoed

class SomeClass {
    // Kotlin allows this, as you can specify the intended type here
    var property: String
}

jwalgemoed

// Classic declaration
ArrayList<Integer> list = new ArrayList<Integer>();

// But honestly, since Java 7 Java we can use the diamond operator <> (or omit it entirely)
ArrayList<Integer> list = new ArrayList();

// Var declaration
var list = new ArrayList<Integer>();

jwalgemoed

// Classic declaration, we are repeating ourselves
Integer ten = new Integer(10);

// New declaration, using 'var' 
var ten = new Integer(10);

jwalgemoed

// We can explicitly work against interfaces
List<String> list = new ArrayList(); // type: List<String>

// Using var, we can't do this without casting
var list = new ArrayList<String>();  // Inferred type ArrayList<String>
var list = (List)new ArrayList<String>(); // Inferred type List<String>

jwalgemoed

// Can't use the local variable type inference with fields
public class SomeClass {
    // Not allowed, var is only available for local variables
    var property = "";
}

// Can't leave values unitialzed
var uninitialized; // No way to infer the type

// Can't just assign null to var to reassign later (but workaround available)

jwalgemoed

var variable = returnValue(); // Type is inferred from the return type of the method

// Method declaration
public String returnValue() {
  return "w00p";
}

jwalgemoed

var strings = Arrays.asList("a", "b", "c");
for(var str: strings) {
  System.out.println(str);
}

jwalgemoed

data class User(val name: String?, val email: String?) {
    fun isValid() = !name.isNullOrBlank() && name.length > 3 
                    && !email.isNullOrBlank() && email.length > 3
}

jwalgemoed

@ExperimentalContracts
fun User?.isComplete(): Boolean {
    contract { returns(true) implies (this@isComplete != null) }
    return this != null && this.isValid()
}

jwalgemoed

contract { 
  returns(true) implies (this@isComplete != null) 
}